It's coolant, and a thick rubber donut goes between the two.

Many will tell you that you need loads of silicone to get it to seal. I've never had a problem when the bolt hole threads are chased and that bolt is tightened first, before tightening down the rest of the intake manifold bolts.

Make sure you chase the threads in the hole, even with a brand new aftermarket manifold. Especially with a brand new afternarket manifold!